Rainfall-induced failures of natural and artificial slopes such as cut slopes, which are subjected to freezing and thawing, have been frequently reported in Hokkaido, Japan. In particular, many failures occur intensively from spring to summer seasons. Despite numerous field studies, explanation of their mechanical behavior based on in situ data has not yet been completely achieved due to the difficulty in grasping failure conditions. This study aims at clarifying the aspects of in-situ volcanic slopes subjected to rainfall and freeze-thaw action. The changes in soil moisture, pore pressure, deformations, and temperatures in the slope were investigated using soil moisture meters, tensiometers, thermocouple sensors, clinometers, settlement gauges, an anemovane, a snow gauge, and a rainfall gauge. The data generated from these measures indicated deformation in the slope examined mainly proceeded during the drainage process according to changes in soil moisture. Based on this data, a prediction method for failures is discussed in detail. 1. Introduction In Hokkaido, Japan, there are over 40 Quaternary volcanoes, and pyroclastic materials cover over 40% of its area. Significant volcanic activities occurred in the Neogene’s Quaternary period, and various pyroclastic materials such as volcanic ash, pumice, and scoria were formed during those eruptions. Such volcanic soils have been used as a useful construction material, especially on foundations or man-made geotechnical structures (embankments and cut slopes, etc.). However, research on volcanic coarse-grained soils from an engineering standpoint is extremely limited in comparison with cohesionless soils (Miura et al. [1]). Recent earthquakes and heavy rainfalls in Hokkaido generated the most serious damage in the ground, natural slopes, cut slopes, and embankments, which are composed of volcanic soils (e.g., JSSMFE [2], JSCE [3]), as seen in the slope failure of a residential embankment due to the 1991 Kushiro-oki earthquake (JSSMFE [2]). Furthermore, cut slope failures attributed to freezing and thawing have also been observed in the Hokkaido expressway in spring and summer seasons. Figure 1 shows the mechanism of frost heaving in cut slope and failure modes in cold regions. In cold regions such as Hokkaido, Japan, slopes freeze from their surface with the formation of ice lenses during the winter season (see Figure 1(a)). Thereafter, the frozen soils thaw gradually from the ground surface until summer season. In the freezing and thawing sequence, the surface layer of a slope may exhibit high moisture
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